Make frosting:
Stir chocolate chips in heavy medium saucepan over very low heat until melted and smooth. Remove from heat and cool to lukewarm. Spoon 1/3 cup chocolate into small bowl and reserve for decoration. Beat margarine and sugar in medium bowl until fluffy. Beat in remaining melted chocolate and orange juice concentrate.

Place 1 cake layer on platter. Spread with 2/3 cup frosting. Top with second cake layer. Spread remaining frosting over top and sides of cake. Arrange additional orange segments around top edge of cake. Rewarm 1/3 cup reserved chocolate to pourable consistency if necessary. Drizzle orange segments with chocolate. (Can be prepared 1 day ahead. Cover with cake dome and refrigerate.)